1. Field of the Invention
The present invention relates to television systems, and more specifically to a method and apparatus for recovering the digital data encoded in a television signal.
2. Related Art
Television systems are generally designed to receive and display images encoded in a television signal. Television signals are typically received in a predetermined format such as composite-sync format well known in the arts. A television signal generally includes display data and corresponding synchronization signals. The display data usually represents color intensity for different points and the synchronization signals provide a time reference such that each point is associated with a point of an image. Synchronization signals typically include horizontal synchronization signals separating each line and vertical synchronization signals separating each frame. Each frame usually corresponds to an image and frames are encoded at 60 Hz in conventional television signals according to NTSC format known in the art.
Digital data is encoded in television signals in many instances. For example, digital data is often encoded in the vertical blanking interval (VBI) of a television signal. VBI generally refers to the time duration or signal portion between frames. The VBI duration provides sufficient time for the scan electronics of a (CRT based) television system to move a scan position to point from the bottom end of a display screen to the top. The television signal corresponding to the VBI period typically does not contain any display data (or image data), and thus a television signal portion corresponding to the VBI period has been conveniently used to encode digital data.
Using the ability to encode digital data in a television system, broadcasters (or television signal generators, in general) may send data corresponding to several applications useful for viewers. For example, teletext is often encoded in the VBI to enable the display of closed-captioning on television displays. Some companies broadcast television guide (indicating the program schedule) and some other companies provide stock quotes and news flashes using VBI portion of a television signal. Digital data can be encoded in television signal portions other than VBI also. For example, an entire channel of a television signal can be used to encode teletext data.
Recovery of the digital data encoded in a received television signal typically entails sampling the television signal using a sampling clock having the same frequency ("encoding frequency") as that at which the digital data is encoded. Assuming for illustration that the encoding frequency of digital data representing teletext is 6.4 MHZ, the received television signal also needs to be sampled using a sampling clock having a frequency of 6.4 MHZ.
In addition, the phase of the sampling clock needs to be adjusted such that each sample is taken around the center of the display data portion corresponding to a digital data element. By taking samples at the center of a display data portion, each sample may be taken during a steady-state (stable) period of the display data portion as is well known in the relevant arts. The steady-state of a display data portion accurately represents the digital data encoded in the corresponding display data portion, and the digital data element encoded in the sampled portion can be accurately recovered.
To enable a sampling clock having a matching frequency and desired phase to be generated, reference (synchronization) signals are often included in a television signal. For example, when encoding teletext data, sixteen pulses of the clock signal used for encoding the teletext data are provided in the television signal.
A typical television system recovering digital data includes a phase-locked-loop (PLL) which generates a sampling clock signal synchronized with the reference signals (associated with the digital data) included in a received television signal. As the digital data can be encoded at high frequencies, it is often necessary that the PLL be accurately synchronized with the received reference signals. PLLs operating at high frequencies and with accuracy usually require crystals (or other hardware) operating with a very low jitter. Typically, such crystals or PLLs are expensive, and the high cost may be undesirable at least in some situations.
Therefore, what is needed is a method and apparatus for recovering the digital data encoded in a television signal in a cost-effective manner.